1. Field of the Invention
This invention relates to copolyesters and copolyesteramides, methods for the preparation of injection molded articles therefrom, and filled compositions thereof and magnetic compositions thereof.
2. Prior Art
First, liquid crystalline polyester and polyesteramide resins are discussed.
Liquid crystalline polyesters can be injection molded into parts having a number of advantageous properties including a high degree of crystallinity, self-reinforcement effect, mechanical strength, a low coefficient of linear expansion, flame retardancy, chemical resistance, solvent resistance, and a low factor of molding shrinkage.
In particular, all aromatic polyesters having a linear skeleton such as a para-phenylene or 4,4'-biphenylene group are very useful thermoplastic resins also exhibiting improved heat resistance. For example, all aromatic liquid crystalline polyesters consisting solely of a para-hydroxybenzoic acid component have a melting point of about 610.degree. C., and all aromatic liquid crystalline polyesters consisting of hydroquinone and terephthalic acid components have a melting point of about 600.degree. C. U.S. Pat. No. 3,637,595 discloses an all aromatic liquid crystalline polyester which consists of terephthalic acid, para-hydroxybenzoic acid, and 4,4'-biphenol components and has a softening point of higher than about 400.degree. C.
Although these liquid crystalline polyesters are heat resistant, their high melting temperature is a problem. For example, it is impossible in a practical sense to injection mold those polyesters having a melting point of higher than about 600.degree. C. Even the polyester disclosed in U.S. Pat. No. 3,637,595 must be molded at a temperature of higher than about 400.degree. C., which causes degradation or coloring of the resin and requires a special molding machine.
A resin is desired to have a lower melting temperature because of ease of injection molding. However, there is the tendency that resins having a lower melting temperature are less heat resistant. Then the melting temperature of a resin cannot be extremely lowered if its heat resistance is taken into account. It is desired that a liquid crystalline polyester can be molded or processed at a melting temperature in the range of from about 300.degree. C. to about 350.degree. C. which is lower than the degradation starting temperature of the polyester.
Several methods are known for lowering the melting temperature of all aromatic liquid crystalline polyesters in order to eliminate the difficulty of molding as disclosed in Jouranl of Applied Polymer Science, Applied Polymer Symposium, 41, pages 25-33 (1985).
(1) Introduction of a substituent into an aromatic ring
A first method is by introducing such a substituent component as 2-chlorohydroquinone, 2-methylhydroquinone, and 2-phenylhydroquinone.
(2) Introduction of a flexural component
A second method is by introducing a component such as resorcin, isophthalic acid, meta-hydroxybenzoic acid or a component such as 4,4'-dihydroxybiphenyl ether, 2,6-naphthalenedicarboxylic acid, 2,6-naphthalene diol, and 2-hydroxy-6-naphthoic acid.
(3) Introduction of an aliphatic component
A third method is by introducing a component such as bisphenol-A and ethylene glycol.
The monomers used in these methods are rather commercially less available except the metaphenylene group-containing components and aliphatic components. A problem still arises with the introduction of a metaphenylene group-containing component. Only a limited amount of metaphenylene group-containing component can be introduced because a polymer having a high proportion of a metaphenylene group-containing component introduced suffers from a low melting temperature, loss of liquid crystallinity, and a markedly reduced mechanical strength. Therefore, introduction of aliphatic components is an ideal method for lowering the melting temperature of all aromatic liquid crystalline polyesters by using commonly available chemical stocks.
Liquid crystalline polyesters having an aliphatic component introduced therein are disclosed in, for example, Japanese Patent Publication Nos. 56-18016 and 59-13531 and Japanese Patent Application Kokai No. 58-84821. The method disclosed in Japanese Patent Publication No. 56-18016 introduces a polyethylene terephthalate (PET) component containing an ethylene glycol unit as an aliphatic component into a para-hydroxybenzoic acid component. The highest mechanical strength is achieved when the polyester composition contains about 40 mol% of the PET component. The mechanical strength is very low with a higher or lower content of the PET component. The polyesters containing about 40 mol% of the PET component, however, have a heat distortion temperature as low as 70.degree. C. or less. When heat resistance is of great interest, the content of PET component may be lowered, but undesirably at the sacrifice of mechanical strength. Therefore, it is impossible for this method to satisfy both mechanical strength and heat resistance. The presence of PET component in a proportion as high as 40 mol% is disadvantageous in manufacture because it takes a long time for polymerization to proceed at a high temperature and a high vacuum until the molecular weight capable of satisfactory performance is reached.
Japanese Patent Publication No. 59-13531 discloses that about 50 mol% of PET component is introduced so that the resulting polyester may exhibit a satisfactory mechanical strength. This polyester is not regarded commercially useful because its heat distortion temperature is lower than 80.degree. C. The increased content of PET component requires an undesirably long time for polymerization to proceed at a high temperature and a high vacuum.
The polyesters disclosed in Japanese Patent Application Kokai No. 58-84821 are destined for the formation of filaments and films. An approach is made so as to optimize the melting temperature of polyesters for filament spinning or film formatiion. These polyesters have less heat resistance and unsatisfactory mechanical strength when they are injection molded.
When liquid crystalline polyesters are destined for the preparation of injection molded products, it is desired that they have a low melting temperature at which injection molding can be readily carried out, that is, a melting temperature of lower than about 350.degree. C., and as high a heat resistant temperature as possible, that is, a heat distortion temperature in excess of about 170.degree. C.
Liquid crystalline polyesteramides are modified liquid crystalline polyesters having amide bonds introduced therein in addition to ester bonds for the purpose of improving the adhesion, fatigue resistance and reducing the anisotropy of the polyesters. The polyesteramides are disclosed in Japanese Patent Application Kokai Nos.
______________________________________ 57-137321, 57-145123, 57-172921, 57-177019, 57-177020, 57-177021, 58-29820, 58-01722, 58-89618, 61-51032, 61-236819, 61-236826, 61-236827. ______________________________________
Introduction of amide bonds, however, induces a reduction in thermal stability (an extended exposure at a high temperature results in a loss of mechanical strength), a reduction in weatherability (particularly a reduction of physical properties under the influence of ultraviolet radiation), and an increase of melt viscosity (reduced fluidity adversely affects moldability). The higher the amide content, the more outstanding is this tendency. As opposed to the fact that some useful liquid crystalline polyesters are commercially available at present, it is difficult to produce commercially useful liquid crystalline polyesteramide because of these drawbacks.
Compositions of polyester and polyesteramide have the following problems.
In general, aromatic polyester resins have drawbacks common to liquid crystalline polyesters, that is, they shows marked anisotropy with respect to mechanical strength, coefficient of linear expansion, and mold shrinkage factor.
Liquid crystalline polyesters have the nature known as anistropy that the strength of a melt molded resin in a machine or oriented direction (MD9 is substantially different from that in a transverse direction (TD). Since failure of an injection molded part by an external force generally takes place at the weakest site, cracks occur in a molded part of liquid crystalline polyester in its transverse direction (TD). To improve the practical strength of liquid crystalline polyester, the anisotropy thereof must be mitigated, that is, the TD strength thereof must be increased.
Another drawback of aromatic polyesters is poor mar resistance.
Liquid crystalline polyesteramide resins are improved in the drawbacks of anisotropy common to aromatic polyesters as previously described, but such an improvement is yet insufficient. They also suffer from a marring problem.
Next, magnetic polyester and polyesteramide compositions are described.
Plastic magnets are inferior in magnetic characteristics to sintered magnets, but have the advantages that a number of products can be readily obtained by injection molding, and they are lightweight and can have a complicated shape. Plastic magnets include magnetic powder and binder resins which are usually epoxy resins and polyamide resins such as nylon-6 and nylon-66. Therefore, the mechanical strength and heat resistance of plastic magnets depend on the particular type of resin used as the binder. For example, plastic magnets based on heat resistant epoxy resins have a heat distortion temperature of from 100.degree. to 120.degree. C. and magnets based on nylon have a heat distortion temperature of from 140.degree. to 160.degree. C.
In order that magnetic resin compositions may find a wider variety of applications, it is important to increase the heat resistance of the compositions. In general, a composition comprising a more heat resistant resin are more difficult to mold, losing the advantage of magnetic resin compositions that a number of parts having a complicated shape can be molded.